Folding Structures Made From Thin Sheet Metal

In the construction sector, the increasing demand for individualized products arising in the last decades is particularly expressed in the so called freeform architecture. With the exception of standardized parts, however, spatially multiple-curved steel components are rarely used in the construction sector, although their high stiffness in combination with a low weight implies a high potential for lightweight design. Due to missing universal and mature construction methods, freeform buildings are mostly realized using customized solutions that oftentimes include material-consuming substructures, whereas the visible skin has neither structural nor functional properties. In this context, the research report presents a new concept for self-supporting lightweight freeform structures that integrates load-bearing, functional properties and façade design in one unit by using a double-layered structure. The combination of tessellation and folding enables the generation of self-supporting structures made of sheet metal and has been applied to complex free formed geometries within the described research project. In order to meet the resulting demand for a large number of individual and complex component geometries, an innovative and flexible manufacturing chain has been developed using incremental sheet forming as a central element.

The production of such a large number of individual parts is a challenging task - a flexible manufacturing process and an optimized overall process chain is necessary. The triangular elements, as well as the star-like connectors, are produced using a CNC laser device. The joining lips are then bended using a manual swing-folding machine. For the most complex parts of the structure - the pyramids - common production methods are inapplicable: a folding process of the pyramidal elements is difficult in terms of three dimensional bending. On the other hand CNC-cutting of flat triangular elements significantly increases the amount of parts and joints. Traditional sheet metal forming processes such as deep drawing are able to produce folded parts out of one single sheet of metal without the need of subsequent joining. However, the need for complex and expensive tooling for these processes is only economical in large batch productions of identical elements. Therefore, an innovative fabrication chain for the pyramidal elements was developed: the basic shapes of the pyramids were formed using ISF, an economical forming process for small batch production and prototypes, which is flexible due to the very low tooling effort. Subsequently, the outer contours are trimmed and bend.

In the ISF process, a CNC-controlled forming tool follows the contour line of the geometry on a flat sheet. After each completed line, the tool performs a vertical step down – the parts are formed without the use of custom fabricated molds. A partial die supports the tops of the pyramids; the resulting identical top is the basis for the star-like joining detail. The contours of the sheets are fixed in a frame of clamping devices, which are following the vertical movement of the process. The non-planar basis of the pyramidal elements therefore has to be extended to a shared plane. Due to the local forming of the sheets, the resulting forces are relatively low. A standard CNC milling machine was modified by adding a basis of clamping devices, each with two degrees of freedom.